The invention relates to coreactive photoinitiators for photopolymerization of systems containing ethylenically unsaturated compounds.
Photochemically induced polymerization reactions have taken on great importance in industry, in particular when rapid hardening of thin layers is important, such as, for example, on hardening of paint and resin coatings on paper, metal and plastic, or on drying of printing inks, since these processes are distinguished compared to conventional methods of printing and coating objects through a saving in raw materials and energy and less environmental pollution. However, the preparation of polymer materials per se through polymerization of appropriate unsaturated monomeric star ting materials is also frequently carried out photochemically, it being possible to use conventional processes such as solution and emulsion polymerization.
Since, in the reactions mentioned, none of the reactants is generally cap able of absorbing the photochemically active radiation to an adequate extent, it necessary to add so-called photoinitiators which are capable of either absorbing incident high-energy radiation, usually UV light, to form active starter radicals, which themselves initiate the photopolymerization, or of transferring the absorbed energy to one of the polymerizable reactants for free-radical formation. The initiators do not normally participate in the actual potymerization reaction.
The major initiators which have hitherto been employed for photopolymerization of unsaturated compounds are benzophenone derivatives, benzoin ethers, benzil ketals, dibenzosuberone derivatives, anthraquinones, xanthones, thioxanthones, .alpha.-haloacetophenone derivatives, dialkoxyacetophenones and hydroxyalkylphenones.
As is known, however, the industrial applicability of many of the substances mentioned is limited, in some cases considerably, by a number of disadvantages. These include, in particular, a frequently unsatisfactory reactivity in the ability to initiate photopolymerization of ethylenically unsaturated compounds. Besides molecule-specific reactivity, the solubility or the ability of the photoinitiators to be incorporated as homogeneously as possible into the photopolymerizable systems frequently plays a crucial role here.
Further problems are the dark-storage stability of the systems to which photoinitiators have been added and the possible influencing of the final product by radicals or degradation products of the photoinitiator. Such radicals can lead to a more or less pronounced effect on the product's properties, depending on the nature and quantity. In photopolymerized paint coatings, the major area of application for photoinitiators, for example, such radicals can affect the final achievable hardness of the coating; in addition, undesired color changes, for example yellowing, can occur, often only after a relatively long time. Initiator radicals or degradation products thereof can become noticeable due to an unpleasant odour as a consequence of their more or less pronounced volatility; their diffusion from the coating into the surrounding media can cause problems, for example in packaging materials which are provided with photopolymerized coatings, such as, for example, cans and tubes for foods. It is precisely in this area of application that the question of applicability is definitively determined by the possible or proven toxicity of the photoinitiators and tile degradation products thereof.
A particular problem, above all with respect to broad application of photoinitiators, is that they can, naturally, only be employed in systems which essentially contain components having olefinic double bonds which can be polymerized by means of free radicals.
Thermocurable systems based exclusively on polyaddition or polycondensation reactions which are not induced by free radicals cannot be converted into radiation-curable systems by adding free radical-forming photoinitiators. Accordingly, the choice of materials for components for radiation-curable systems is limited. Many material-specific properties of thermocurable systems cannot or cannot yet be used in radiation-curable systems without further action. One alternative here is provided by so-called hybrid binder systems, in which thermocurable and photochemically curable components are combined and in which the thermal and photochemical reactions can take place simultaneously or successively. However, compatibility problems, in particular with respect to the photoinitiators to be employed frequently arise during development of such systems. Thus there continued to be a particular demand among experts for photoinitiators which, besides excellent initiator properties and good dark storage stability of the systems to which they have been added, also have a broad applicability, even in systems with a complex composition, and which can themselves, or their photolysis products, be bound in such systems in a migration-resistant manner.
Individual steps in this direction have already been taken. Thus, for example, German Offenlegungsschrift 3,534,645 and European Offenlegungsschrift 161,463 describe photoinitiators of the hydroxyalkylphenone type which carry specifically olefinically unsaturated substituents. These initiators or their photolysis products can be bound into the polymer composition by copolymerization with the components of the radiation-curable system. They can alternatively initially be thermally polymerized themselves and then, as polymeric and as migration-resistant photoinitiators, introduced into the radiation-curable system. However, these specific copolymerizable or polymeric photoinitiators have an only limited range of applications.